The invention is directed to an acoustic scanning microscope which utilizes reciprocal effects in the acoustic near-field of an object. In this context, the expression "near-field" signifies that the size of the radiation source and its distance from the object are approximately the same and significantly smaller than the wavelength of the radiation used. An important feature in a near-field arrangement is that the resolution is no longer classically limited by the wavelength of the radiation used, but instead can be much smaller, and is determined by the size and distance of the radiation source. This is known, e.g., for an optical near-field scanning microscope discussed in European Published Patent Specification EP-B2 112,401.
An acoustic scanning microscope which undershoots the classical resolution limit and achieves a resolution of approximately one tenth of the sound wavelength is discussed by J. K. Zieniuk and A. Latuszek in IEEE Ultrasonics Symposium 1986 at pages 1037-1039.
Ultrasonic waves of approximately 3 MHz are transmitted through the object (coupled via a fluid coupling medium to a sound waveguide held at a short distance above the object) and fed from the latter to a transducer serving as a detector. A conical frustum shape of the sound waveguide, whose area of coverage of approximately 20 .mu.m in diameter is distinctly smaller than the sound wavelength used (approximately 0.5 mm), faces the object. The size of the surface area covered determines the achievable resolution.
Such a microscope is also discussed in U.S. Pat. No. 4,646,573.
A transmission arrangement of this type is basically suitable only for a narrow class of objects.
A reflex arrangement with a conical frustum instead of an acoustic lens is possible in theory. However, in such an arrangement there occurs high power losses, the necessity for pulse operation, and substantial disturbances due to various reflexes, all of which have occurred to a lesser extent in the classical acoustic microscope (see A. Thaer, M. Hoppe, W. J. Patzelt, Leitz Mitt. Wiss. Techn. VIII (1982), pages 61-67).
A power microscope is discussed in European Published patent Specification EP-A2 223,918. This microscope acquires a topographic image of an object by bringing up a point so close to the object that the electron clouds of the atoms in the point and in the object overlap, so that interatomic forces occur. This distance is less than 1 nm. The point is attached to a bar spring, and forces are detected through the influence of the spring deflection on a scanning tunnelling microscope observing the spring. In this regard, it is proposed to cause the bar spring to oscillate at its natural frequency of 2 KHz and at an amplitude smaller than 1 nm in the z direction. This introduces a carrier frequency response which reduces the effects of errors. With this power microscope it is especially advantageous to locate the object and the detection system in an ultra high vacuum (approximately 10.sup.-8 Pa).
German Publication DE 3,723,933 A1 (which was not published prior to development of the instant invention) discusses a contact detector for contact surfaces in the region of about one square micrometer and contact forces in the region of less than one millinewton. In this device, a bar-shaped resonator with a pointed probe is excited to have natural mechanical oscillations in the longitudinal direction and a contact signal appears if during the approach of the probe to the measurement surface the amplitude falls by, e.g., a decibel. Frequency variations can also be detected. It is possible for a surface to be scanned and the amplitude to be constantly regulated in the process. Separate pairs of electrodes are provided for exciting and detecting oscillations of the resonator. Detuning is achieved through direct mechanical contact without a coupling medium, as occurs in the case of hardness tests. Characteristics of the acoustic field, including the acoustic near-field, are not discussed.